Disentangling the impacts of heat wave magnitude, duration and timing on the structure and diversity of sessile marine assemblages

Fouling community shows high resistance and metabolic resilience towards experimental high intensity heatwave

Climate change is predicted to increase the prevalence of marine heatwaves with an increase in heatwave frequency and intensity. While some studies have shown the effect of marine heatwaves in warm temperate climates and the effect of overall higher temperature in warm and cold temperate climates, it is yet not entirely understood how heatwaves impact marine urban communities in cold temperate climates. As thermal resistance might be related to selective pressures and acclimation, it seems reasonable to assume that they may have a strong impact on local fauna and flora. In the present study, we simulated an in situ high amplitude heatwave and observed the community structure and the metabolism of Bugula neritina at two time-steps after the heatwave and compared them to control communities and individuals. Contrary to our expectations, the community structure remained vastly unaffected, as did the total metabolome of B. neritina. This shows that the community was able to resist the disturbance of the heatwave. Bugula neritina additionally showed a certain metabolic resilience as the already minor differences in the metabolome between control and the heatwave treatment diminished even further between the tested time steps.

The Young and the Resilient: Investigating Coral Thermal Resilience in Early Life Stages

Global ocean warming is affecting keystone species distributions and fitness, resulting in the degradation of marine ecosystems. Coral reefs are one of the most diverse and productive marine ecosystems. However, reef-building corals, the foundational taxa of coral reef ecosystems, are severely threatened by thermal stress. Models predict 40-80% of global coral cover will be lost by 2100, which highlights the urgent need for widespread interventions to preserve coral reef functionality. There has been extensive research on coral thermal stress and resilience, but 95% of studies have focused on adult corals. It is necessary to understand stress during early life stages (larvae, recruits, and juveniles), which will better inform selective breeding programs that aim to replenish reefs with resilient stock. In this review, we surveyed the literature on coral thermal resilience in early life stages, and we highlight that studies have been conducted on relatively few species (commonly Acropora spp.) and in limited regions (mainly Australia). Reef-building coral management will be improved by comprehensively understanding coral thermal resilience and fitness across life stages, as well as in diverse species and regions.

Large potential impacts of marine heatwaves on ecosystem functioning

Ocean warming is driving significant changes in the structure and functioning of marine ecosystems, shifting species' biogeography and phenology, changing body size and biomass and altering the trophodynamics of the system. Particularly, extreme temperature events such as marine heatwaves (MHWs) have been increasing in intensity, duration and frequency. MHWs are causing large-scale impacts on marine ecosystems, such as coral bleaching, mass mortality of seagrass meadows and declines in fish stocks and other marine organisms in recent decades. In this study, we developed and applied a dynamic version of the EcoTroph trophodynamic modelling approach to study the cascading effects of individual MHW on marine ecosystem functioning. We simulated theoretical user-controlled ecosystems and explored the consequences of various assumptions of marine species mortality along the food web, associated with different MHW intensities. We show that an MHW can lead to a significant biomass reduction of all consumers, with the severity of the declines being dependent on species trophic levels (TLs) and biomes, in addition to the characteristics of MHWs. Biomass of higher TLs declines more than lower TLs under an MHW, leading to changes in ecosystem structure. While tropical ecosystems are projected to be sensitive to low-intensity MHWs, polar and temperate ecosystems are expected to be impacted by more intense MHWs. The estimated time to recover from MHW impacts is twice as long for polar ecosystems and one-third longer for temperate biomes compared with tropical biomes. This study highlights the importance of considering extreme weather events in assessing the effects of climate change on the structures and functions of marine ecosystems.

Impact of in Situ Simulated Climate Change on Communities and Non-Indigenous Species: Two Climates, Two Responses

Climate change constitutes a major challenge for marine urban ecosystems and ocean warming will likely strongly affect local communities. Non-Indigenous Species (NIS) have been shown to often have higher heat resistance than natives, but studies investigating how forthcoming global warming might affect them in marine urban environments remain scarce, especially in Situ studies. Here we used an in Situ warming experiment in a NW Mediterranean (warm temperate) and a NE Atlantic (cold temperate) marina to see how global warming might affect recruited communities in the near future. In both marinas, warming resulted in significantly different community structure, lower biomass, and more empty space compared to control. However, while in the warm temperate marina, NIS showed an increased surface cover, it was reduced in the cold temperate one. Metabolomic analyses on Bugula neritina in the Atlantic marina revealed potential heat stress experienced by this introduced bryozoan and a potential link between heat stress and the expression of a halogenated alkaloid, Caelestine A. The present results might indicate that the effects of global warming on the prevalence of NIS may differ between geographical provinces, which could be investigated by larger scale studies.

Effects of Field Simulated Marine Heatwaves on Sedimentary Organic Matter Quantity, Biochemical Composition, and Degradation Rates

Simple Summary

Marine heatwaves (MHWs) are intensifying due to global warming. Based on their effects on biochemical reactions, they are also likely to affect coastal biogeochemistry. We investigated organic matter quantity, composition and degradation rates in nearshore sediments affected by simulated MHWs, with 1.5 and 5.0 °C anomalies, before and after 3 and 11 weeks from the release of an artificial warm water plume. MHWs enhanced organic loads (by >100%), with larger effects in the short-term under the highest temperature anomaly. Phytopigment contents increased (by 50–90%) in the short term but decreased to initial values in the longer one. The autotrophic and lipid contents decreased with time (by 15–50% 53–79%, respectively), suggesting a drop in the nutritional quality of organic matter, along with a slowdown of its turnover. We contend that MHWs’ intensification will affect not only species and communities but will also alter sediment biogeochemistry and, possibly, the energy transfer towards higher trophic levels.

Abstract

Since rising temperature (T) will enhance biochemical reactions and coastal marine sediments are hotspots of carbon cycling, marine heatwaves’ (MHWs’) intensification caused by climate change will affect coastal biogeochemistry. We investigated the effects of MHWs on sediment organic matter (OM) in a nearshore locality (NW Sardinia, Mediterranean Sea) receiving an artificial warm water plume generating T anomalies of 1.5–5.0 °C. Sediments were collected before and after 3 and 11 weeks from the initial plume release. Both MHWs influenced sedimentary OM quantity, composition, and degradation rates, with major effects associated with the highest T anomaly after 3 weeks. Both MHWs enhanced sedimentary OM contents, with larger effects associated with the highest T anomaly. Phytopigment contents increased in the short term but dropped to initial levels after 11 weeks, suggesting the occurrence of thermal adaptation or stress of microphytobenthos. In the longer term we observed a decrease in the nutritional quality of OM and a slowdown of its turnover mediated by extracellular enzymes, suggestive of a decreased ecosystem functioning. We anticipate that intensification of MHWs will affect benthic communities not only through direct effects on species tolerance but also by altering benthic biogeochemistry and the efficiency of energy transfer towards higher trophic levels.

Capacity of an ecologically key urchin to recover from extreme events: Physiological impacts of heatwaves and the road to recovery

Heatwaves are increasing in frequency and intensity, with substantial impacts on ecosystems and species which maintain their function. Whether or not species are harmed by heatwave conditions by being pushed beyond their physiological bounds can depend on whether energy replacement is sufficient to enable recovery from acute stress. We exposed an ecologically important sea urchin, Heliocidaris erythrogramma, to experimental marine heatwave scenarios in context with recent summer heat anomalies in moderate (25 °C), and strong heatwave (26 °C) conditions for 10 days, followed by a 10-day recovery period at normal summer temperature (23 °C). Greater heatwave intensity drove higher metabolic rates which were not matched with a concurrent increase in food consumption or faecal production. However, food consumption increased during the post-heatwave recovery period, likely to replenish an energy deficit. Despite this, mortality increased into the recovery period and seemed to be caused by latent effects, manifesting as a decline in health index as individuals progressed from spine and pedicellariae loss, through to loss of tube foot rigor, bald patch disease, culminating in mortality. We show for the first time that the acute thermal stress of heatwaves can have latent physiological effects that cause mortality even when conditions return to normal. Our results show that the negative effects of heatwaves can manifest after relief from stressful conditions and highlight the importance of understanding the latent effects on physiology and health. This understanding will offer insights into the long-term potential for stress recovery following seemingly sublethal effects and whether the restoration of ambient conditions post-heatwave is sufficient to ensure population stability.

Benthic estuarine communities' contribution to bioturbation under the experimental effect of marine heatwaves

Marine heatwaves are increasing worldwide, with several negative impacts on biological communities and ecosystems. This 24-day study tested heatwaves' effect with distinct duration and recovery periods on benthic estuarine communities' diversity and contribution to ecosystem functioning experimentally. The communities were obtained from a temperate estuary, usually subjected to high daily thermal amplitudes. Our goal was to understand the communities' response to the thermal change, including the community descriptors and behavioural changes expected during heat extremes. We measured community composition and structural changes and the bioturbation process and nutrient release as ecosystem functioning measurements. Overall, our findings highlight the potential tolerance of studied estuarine species to the temperature ranges tested in the study, as community composition and structure were similar, independently of the warming effect. We detected a slight trend for bioturbation and nutrient release increase in the communities under warming, yet these responses were not consistent with the heatwaves exposure duration. Overall, we conclude on the complexity of estuarine communities' contribution to functioning under warming, and the importance of scalable experiments with benthic organisms' responses to climate variability, accommodating longer time scales and replication. Such an approach would set more efficient expectations towards climate change mitigation or adaptation in temperate estuarine ecosystems.

Rhodolith primary and carbonate production in a changing ocean: The interplay of warming and nutrients

Rhodolith beds, like many other marine ecosystems, are affected by climate change that is causing an increase in the magnitude and frequency of extreme high temperature events (heat waves). Unfortunately, this does not represent the sole peril for these communities, as coastal urbanization in conjunction with altered precipitation patterns can increase terrestrial-derived nutrient input. In Brazil, rhodolith beds are among the most extensive coastal benthic ecosystems, but despite their vast distribution and great ecological and economic importance, studies on the productivity of these communities and the impact of changing environmental conditions are almost non-existent. This study addressed the individual and combined effects of increases in temperature and nutrient concentration on the physiological performance of two widely distributed rhodolith species, Lithothamnion crispatum and Melyvonnea erubescens. The results showed species-specific responses in net photosynthetic performance, with no response in L. crispatum, while M. erubescens responded negatively to both increase in temperature and nutrients. In contrast, calcification in both species showed a significant decline at high temperature. No interactive effects were found between temperature and nutrients, yet their combined negative effects were additive, resulting in negative daily-integrated net productivity and a large decline in daily carbonate production in both species. This has strong implications for rhodolith bed primary productivity and carbonate production, as heat waves may potentially cause a strong decline in carbonate production (ca. 50% loss), accompanied by a severe drop in primary productivity that will be even more pronounced under high-nutrient conditions. Also, the species-specific responses to changes in temperature and nutrient concentration suggest that the magnitude of impact of these factors on rhodolith bed productivity will depend on the species dominating the community and may finally result in changes in rhodolith community composition.

Comparative transcriptional profiling analysis of the effect of heat waves during embryo incubation on the hatchlings of the Chinese soft-shelled turtle (Pelodiscus sinensis)

Temperature is one of most the important environmental factors that affect the ontogenesis of organisms. In this study, we incubated Chinese soft‐shelled turtle eggs at 28°C (control temperature, C treatment), a temperature with a 16°C cold shock and a 36°C heat shock twice per week (S treatment) or a ramp‐programmed temperature of 29 ± 9°C (with 12 hr (+) and 12 hr (−) every day) (F treatment). The incubation period, hatching success, hatchling weight, and locomotor performance were significantly different between the controls and the different heat treatment groups. The pathogen challenge results illustrated that hatchlings from the S treatment group were more resistant to bacterial infection, whereas hatchlings from the F treatment group were more vulnerable. We used RNA‐seq quantification analysis to identify differentially expressed genes (DEGs) of hatchlings in the S treatment group. Based on the functional annotation results for the DEGs, 9 genes were chosen to verify the RNA‐seq results. The background expression of DEGs was also analyzed for the three treatments, as was the regulation of the pathogen challenge. The results showed that 8 DEGs were related to the immune response after pathogen challenge and that temperature was an important factor in differential regulation of the immunity pathways.

Longer and more frequent marine heatwaves over the past century

Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.

Community responses to seawater warming are conserved across diverse biological groupings and taxonomic resolutions

Temperature variability is a major driver of ecological pattern, with recent changes in average and extreme temperatures having significant impacts on populations, communities and ecosystems. In the marine realm, very few experiments have manipulated temperature in situ, and current understanding of temperature effects on community dynamics is limited. We developed new technology for precise seawater temperature control to examine warming effects on communities of bacteria, microbial eukaryotes (protists) and metazoans. Despite highly contrasting phylogenies, size spectra and diversity levels, the three community types responded similarly to seawater warming treatments of +3°C and +5°C, highlighting the critical and overarching importance of temperature in structuring communities. Temperature effects were detectable at coarse taxonomic resolutions and many taxa responded positively to warming, leading to increased abundances at the community-level. Novel field-based experimental approaches are essential to improve mechanistic understanding of how ocean warming will alter the structure and functioning of diverse marine communities.